Inkjet printing is typically done by either drop-on-demand or continuous inkjet printing. In drop-on-demand inkjet printing ink drops are ejected onto a recording medium using a drop ejector including a pressurization actuator (thermal or piezoelectric, for example). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the recording medium and strikes the recording medium. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image.
Motion of the recording medium relative to the printhead during drop ejection can consist of keeping the printhead stationary and advancing the recording medium past the printhead while the drops are ejected, or alternatively keeping the recording medium stationary and moving the printhead. This former architecture is appropriate if the drop ejector array on the printhead can address the entire region of interest across the width of the recording medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead drop ejector array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a medium advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the medium advance direction as the drops are ejected from the nozzles. After the carriage-mounted printhead has printed a swath of the image while traversing the print medium, the recording medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
A drop ejector in a drop-on-demand inkjet printhead includes a pressure chamber having an ink inlet for providing ink to the pressure chamber, and a nozzle for jetting drops out of the chamber. Two side-by-side drop ejectors are shown in prior art FIG. 1 (adapted from U.S. Pat. No. 7,163,278) as an example of a conventional thermal inkjet drop-on-demand drop ejector configuration. Partition walls 20 are formed on a base plate 10 and define pressure chambers 22. A nozzle plate 30 is formed on the partition walls 20 and includes nozzles 32, each nozzle 32 being disposed over a corresponding pressure chamber 22. Ink enters pressure chambers 22 by first going through an opening in base plate 10, or around an edge of base plate 10, and then through ink inlets 24, as indicated by the arrows in FIG. 1. A heating element 35, which functions as the actuator, is formed on the surface of the base plate 10 within each pressure chamber 22. Heating element 35 is configured to selectively pressurize the pressure chamber 22 by rapid boiling of a portion of the ink in order to eject drops of ink through the nozzle 32 when an energizing pulse of appropriate amplitude and duration is provided.
Other types of actuators that use resistive heating elements to selectively pressurize the pressure chamber for drop ejection include thermal actuators that have a multi-layer cantilevered element that is caused to rapidly bend toward the nozzle when the resistive heating element layer is pulsed. Less heating of the ink is required than for thermal inkjet, where the ink is locally vaporized to provide the ejection pressure.
Uniformity in the volume of the ejected drops of ink in drop-on-demand inkjet printheads is important for providing excellent print quality. If drops are larger or smaller than the nominal value they can cause darker or lighter regions respectively in the printed image, resulting in undesirable banding defects. Drop volume variations can be caused by geometrical variations in the drop ejectors, such as variations in the nozzle diameter. Drop volume variations can also be caused by electrical performance variations, due to variations in heating element resistance R, for example. When a constant voltage V is pulsed across an array of heating elements, heating elements with lower resistance will receive an increased amount of heating power, as the heating power applied to each heating element equals V2/R.
Drop ejectors in drop-on-demand inkjet printheads work well within a given temperature range. Printhead temperature can vary due to variation in ambient temperature as well as to temperature rise associated with the energy dissipated on the printhead during operation. A known problem in drop-on-demand inkjet printing is the degradation in output print quality due to temperature-related changes in the volume of ink that is ejected. One reason why the size of ejected drops increases with temperature of the printhead is that ink viscosity decreases with increased temperature. In addition, for thermal inkjet printheads, the amount of ink that is vaporized by a resistive heating element during a printing pulse increases with increased printhead temperature. Although a significant portion of the heat is carried off by the ejected ink drops, some of the heat remains in the printhead and results in an increased printhead temperature. At sufficiently high temperature the drop ejection can become unreliable, resulting in missing dots in the printed image.
Various printhead temperature control and pulse waveform control systems and methods are known in the prior art for sensing inkjet printhead temperature and using sensed temperature signals to compensate for temperature fluctuations. The approach in printhead temperature control is to keep the printhead within a narrow temperature range by auxiliary heating or cooling for example. In pulse waveform control the approach is to tailor the pulses that are provided to the resistive heating elements in order to compensate for temperature changes on the printhead so that the drop volume remains substantially constant. In both approaches it is important to have an accurate measurement of temperature in the vicinity of the drop ejectors.
U.S. Pat. No. 7,163,278 discloses a thermal inkjet printhead temperature control system that regulates the temperature of a printhead using a temperature sensing device and a heating component. The temperature sensing device includes either a collection of transducers or a single thermistor located on the drop ejector substrate or on a printed circuit board to which the printhead is attached. It is disclosed that at low temperatures, low energy pulses are sent to a nozzle to heat it. These low energy pulses have insufficient energy to cause a drop of ink to be ejected. It is further disclosed that at high temperature, the use profile and the temperature are monitored to see if a particular nozzle exceeds its operable range. If so, printing by that nozzle is stopped until the temperature drops.
As indicated above, drop volume tends to increase with increased temperature of the printhead and ink. It is also known that drop volume can be affected by the pulse waveform. As disclosed in U.S. Pat. No. 4,982,199, ink in the vicinity of the nozzle of a drop ejector can be pre-warmed by pulsing the resistive heating element using one or more pulses that have insufficient energy to form a vapor bubble of ink prior to the firing pulse that forms the vapor bubble. By pre-warming the ink, more of the ink in the nozzle region is brought to the vaporization temperature by the firing pulse before the transfer of heat to the ink from the resistive heating element is interrupted by the formation of the vapor bubble. Vaporizing more of the ink forms a larger bubble, which provides the power for ejecting a larger drop of ink. U.S. Pat. No. 4,982,199 contemplates the use of pre-warming pulses for use in gray-scaling rather than for compensation of drop volume for temperature variation.
U.S. Pat. No. 5,036,337 discloses attaching a temperature sensor to a surface of the drop ejector substrate. The resistive heating elements on the drop ejector substrate are connected to drivers that are not on the drop ejector substrate. Temperature signals from the temperature sensor are sent to a controller, and the controller enables actuation of selected resistive heating elements through the drivers using packets of electrical pulses. A digital clock signal is also provided to the controller. It is disclosed that pulse widths, idle times between pulses or number of pulses per packet can be increased or decreased by one or more clock units to change the pulse waveform in order to control drop volume in response to the temperature measured by the temperature sensor according to a look-up table that provides data to the controller. U.S. Pat. No. 5,917,509 discloses one or more precursor pulses (or warming pulses) that are applied to the resistive heating element for warming the ink nearby, followed by a print pulse that causes a drop of ink to be ejected.
U.S. Pat. No. 5,107,276 discloses a thermal inkjet printer having a printhead that is maintained at a substantially constant operating temperature during printing. Heating elements identified by the printing data for printing are provided with ejecting pulses having sufficient energy to form ink vapor bubbles for ejecting ink drops. To prevent printhead temperature fluctuations during printing, the heating elements not being used to eject droplets are selectively energized with energy pulses having insufficient magnitude to vaporize the ink. In one embodiment a device is provided for determining the logical complement of the printing data, thereby identifying the heating elements not being used to eject droplets. In response to the logical complement input, a sub-threshold pulse width controller can be used to pulse the non-ejecting heating elements with pulses that are too short for ejecting drops, but sufficient to reduce temperature fluctuations on the printhead by providing supplemental heat.
U.S. Pat. No. 5,168,284 discloses a method and apparatus for real-time control of the temperature of thermal inkjet printheads through the use of nonprinting pulses. A closed-loop system includes a closed-loop pulse generator that produces non-printing pulses in response to a difference between a reference temperature signal and a printhead temperature signal produced by a temperature sensor on the printhead. When the printhead temperature exceeds the temperature indicated by the reference temperature signal, the closed-loop system reduces the amount of energy transmitted by the closed-loop non-printing pulses. An open-loop system transmits non-printing pulses to the printhead for each printing interval that the printer does not eject a drop. During each interval, an open-loop pulse generator applies either a printing pulse or one or more nonprinting pulses across a firing resistor. A data interpreter reads print data. If the print data contains a command in a particular printing interval, the data interpreter instructs the open-loop pulse generator to generate a printing pulse. Otherwise the data interpreter instructs the open-loop pulse generator to generate one or more open-loop non-printing pulses. A summing node merges the output of the various pulse generators onto a single trace leading to a firing resistor.
U.S. Pat. No. 5,736,995 discloses a technique for controlling print quality in an inkjet printer by delivering synchronized heating non-printing pulses and printing pulses to the ink firing resistors. A temperature of the printhead substrate is measured and compared against a reference temperature during printing operations. If the measured temperature is below the reference temperature, then the printhead substrate is heated during the printing operations to bring the substrate up to the reference temperature. The heating is done by delivering synchronized heating non-printing pulses and printing pulses to the ink firing resistors during selected print firing periods, such that either the heating pulses or the printing pulses, but not both, occur during a selected print firing period. The heating pulses are logically OR-ed with the printing pulses to achieve the synchronization.
U.S. Pat. No. 6,302,507 discloses an inkjet printhead assembly that includes a data processor, such as a distributive processor having digital circuitry. The data processor formulates firing and timing operations based on sensed information, such as sensed printhead temperature and sensed power supplied, as well as given operating information, such as optimal operating ranges e.g., temperature ranges and energy ranges. The data processor can calculate an adjusted pulse width using a pulse width adjustment factor determined using the sensed temperature along with an equation or look-up table. A printhead memory device included in the printhead assembly can store various printhead specific data including printhead characterization data. A factory calibration process is disclosed for compensating for variations in the printhead assembly, such as variation between ink ejection elements. A turn-on voltage can be determined and used for calculating an operating voltage and nominal pulse width that can be written to the printhead memory. The operating voltage and nominal pulse width calibration data are read from printhead or printer memory.
U.S. Pat. No. 6,322,189 discloses an apparatus and method for controlling temperature fluctuations between printhead dies in a multiple printhead die printer. By reducing temperature variations, changes in image intensity that are attributable to temperature variations are reduced. Temperature control logic is preferably provided in each printhead. By sending a pulse having a reduced duration or reduced current that is insufficient to expel ink, the ink in the printhead can be heated to a desired temperature.
Despite the previous advances in temperature control and drop volume control on inkjet printheads, what is still needed are printing apparatus designs and printing methods that provide drop volume control at high speed for fast throughput printing. If the heating pulse is allowed to overlap with the firing pulse, excessive heating of the resistor can result. To avoid overlap, the summing method disclosed in '284 and the OR-ing method described in '995 referenced above, for example, can require an increase in the firing time interval that results in a reduced printing throughput speed. Additionally still needed are printing apparatus designs and printing methods that enable improved image quality despite changes in drop volume.